The present invention relates generally to rolled steel products and more particularly to a hot-rolled, high strength low alloy steel strip.
The hot rolling of steel strip is performed in a rolling mill containing a series of rolling stands through which passes the hot steel product undergoing rolling. The initial group of rolling stands is the roughing train in which the steel product undergoes reduction to a strip of intermediate thickness. The final group of rolling stands is the finishing train in which the strip is reduced to its final thickness. Each rolling stand includes at least two rolls maintained a predetermined distance apart and between which passes the product undergoing rolling.
Hot rolled, high strength, low alloy steel strip has many uses, particularly in the automotive industry because automobile parts fabricated from this strip have relatively high strength properties at a relatively reduced thickness, and reducing the thickness of a part reduces its weight. A reduction in weight reduces the fuel consumption of an automobile made from these parts. When the yield strength exceeds 550 MPa or 80,000 psi (80 ksi), it can be readily utilized by the automotive industry in thicknesses down to 0.065 in. (1.65 mm) or less.
Most commercial hot rolled steel strips meeting the requirements described in the preceding sentence employ columbium and/or vanadium, both in relatively small quantities, together with a relatively high silicon content (at least 0.2 wt.%).
Examples of prior art patents which disclose embodiments of hot-rolled steel strip having high strength, low alloy characteristics include Korchynsky et al. U.S. Pat. No. 3,666,570, which discloses the use of either columbium or vanadium for strengthening purposes. Hamburg et al. U.S. Pat. No. 4,033,789 and Matas et al. U.S. Pat. No. 3,997,372 each disclose the use of both columbium and vanadium together but also employs a relatively high silicon content (at least 0.2 wt.%). Abraham et al. U.S. Pat. No. 4,142,922 discloses a hot-rolled steel strip employing both columbium and vanadium, together with a relatively low manganese content (e.g., 0.3-0.5 wt.%). Aronson et al. U.S. Pat. No. 3,661,537 discloses a welded pipe structure composed of steel containing columbium plus vanadium but requires a relatively high carbon content (e.g., 0.1-0.2 wt.%).
The hot-rolling of light gauge, high strength, low alloy steel strip is very difficult because very high roll separating forces and high rolling mill motor electric currents are experienced in the finishing train of rolling stands. Columbium, vanadium and silicon, while strengthening the steel, all contribute to the problem described in the preceding sentence. In order to make practicable the production of light gauge, high strength, hot rolled steel strip, the steel composition must be controlled so as to minimize rolling mill loads while attaining the desired strength levels in the finished, hot rolled steel product.
Another problem in hot rolled, high strength, low alloy steel strip is the presence of sulfide inclusion which, unless eliminated or shape-controlled, have an adverse effect on the properties of the hot rolled steel strip.